Data migration method for disk apparatus

ABSTRACT

Data migration can be executed between new and old disk apparatuses without changing the disk definition of a host computer. A switch having the function of online data migration is provided between the host computer and the old disk apparatus, data are copied to the new disk apparatus while the disk apparatus is accessed continuously by the host computer. If a SCSI command for identifying disk apparatuses is issued by the host computer after data migration, the response of the old disk apparatus is sent back.

The present application is a continuation of application Ser. No.10/661,574, filed Sep. 15, 2003; which claims priority of JapanesePatent Application Serial Number 2003-178976, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a computer system and a method forcontrolling interface commands of a disk apparatus for online datamigration technology of the disk apparatus and, more particularly, tomigration technology for the contents of the disk apparatus for storingtask process data while a host computer continues to process task data.

2. Description of the Related Art

Conventionally, in the case of updating a newly connected disk apparatus(new disk apparatus) from a disk apparatus (old disk apparatus ormigration source disk apparatus) functioning as an outside storagedevice, in order to utilize continuously data that are processed up tonow, data of the old disk apparatus shall be moved to the new diskapparatus (migration). In such a case, as a conventional general method,a method of storing the contents of the current disk apparatus once in abackup file of a tape unit or the like and then restoring after the diskapparatus was exchanged has been employed. Otherwise, there has beenknown a method for connecting the new disk apparatus, as well as aconventional old disk, to a host computer and then storing copy data ofthe old disk apparatus in the new disk apparatus by the host computer.Here, these methods stop the tasks of the host computer for a longperiod of time. Operation for stopping tasks for such a long period oftime cannot be accepted as the tasks of the most modern data centercharacterizing non-stop operation.

For a method for improving such a condition, a patent document 1(Japanese published unexamined patent application No. 2001-249853)discloses a method for executing data migration from the old diskapparatus to the new disk apparatus by connecting the new disk apparatusto a switch provided between the host computer and the old diskapparatus. According to this data migration method, a unit for copyingdata from the old disk apparatus to the new disk apparatus while thedisk apparatus of the host computer is continuously accessed is providedin the switch (or a storage system). However, in this data migrationmethod, the old disk apparatus and the host computer that is used arenecessarily disconnected for a while and then the switch having theabove cited online data migration function shall be provided. Here, ifthe number of access paths between the host computer and the old diskapparatus is one, the computer system shall be halted.

For another method, a patent document 2 (Japanese published unexaminedpatent application No. 11-184641) discloses a method for executing datamigration without stopping while plural access paths are includedbetween the host computer and the disk apparatus and at least one accesspath is continuously maintained. This data migration method is made andused by a changing-over path function provided on the host computer,i.e., a method for changing-over access paths dynamically by managingtwo or more access paths between the host computer and the diskapparatus on the host computer. The changing-over path function of thehost computer, however, may not work properly for a disk apparatushaving different interface specification as a process task of aspecified disk apparatus. The reason is because a SCSI response of thedisk apparatus is utilized inherently in order to identify automaticallyplural access paths for the disk apparatus (referred to a logical unitin the SCSI code). Namely, for such a response, there may be caused aproblem in which access is denied if consistency is not recognized indisk inherent information for an interface command.

[Patent document 1]

Japanese published unexamined patent application No. 2001-249853

[Patent document 2]

Japanese published unexamined patent application No. 11-184641

BRIEF SUMMARY OF THE INVENTION

In the above cited conventional technology, at the time of changing-overan old disk apparatus to a new disk apparatus, since consistency betweenold disk (apparatus) inherent information maintained by a host computeras configuration information and new disk (apparatus) inherentinformation is not firmly assured, the host computer closes an accesspath by judging that the new disk apparatus fails due to thisinconsistency, thus a problem in which the computer system goes down iscaused as a result.

The object of the present invention is to provide interface commandcontrol technology (control system and computer system) of datamigration of the disk apparatus capable of executing non-stop datamigration without requiring stop operation of accessing the diskapparatus by the host computer due to the above cited inconsistency inaccordance with data migration procedures from old disk apparatus to thenew disk apparatus.

Another object of the present invention is to provide interface commandcontrol technology for data migration capable of updating (exchanging) adifferently specified disk apparatus by avoiding causing failure due toa disk inherent information change in accordance with migration from theold disk apparatus to the new disk apparatus.

Another object of the present invention is to provide interface commandcontrol technology for data migration of a computer system having highavailability and maintaining redundancy of the access path by performingcorrectly an alternate path function during and after data migration.

Still another object of the present invention is to provide interfacecommand control technology for data migration of the disk apparatuscapable of executing data migration smoothly without stopping migrationfor the disk apparatus shared by plural host computers.

Still another object of the present invention is to provide the computersystem adapting the interface command control method of the above citeddisk apparatus.

In order to solve the above cited objects, the present invention, in amethod for controlling an interface command of a magnetic disk apparatusof a computer system including one or more host computers, a diskapparatus (old disk apparatus) connected prior to the host computers,and a disk apparatus (new disk apparatus) newly connected to the hostcomputers via a switch, includes the steps of changing-over andconnecting the old disk apparatus to the host computers via the switchbeing connected to the new disk apparatus, executing data migration fromthe old disk apparatus to the new disk apparatus via the switch,identifying a command for inquiring disk identification as an interfacecommand from the host computers and a command for inputting andoutputting data, and sending the command for inquiring the diskidentification to the old disk apparatus.

Further, data migration from the old disk apparatus to the new diskapparatus is executed by an online data migration function of theswitch.

Furthermore, the old disk apparatus and the new disk apparatus operateby a SCSI command from one or more host computers, and a SCSI commandutilized by the host computers for identifying the disk apparatusexchanges data so as to utilize as the same disk apparatus before andafter data migration.

Furthermore, one or more host computers share at least one old diskapparatus, and the old disk apparatus is reused for storing data afterdata migration.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the detaileddescription taken in conjunction with the accompanying drawings and thusare not limited to the present invention in which:

FIGS. 1A and 1B are a block diagram of a computer system for explainingdata migration embodiment of the present invention;

FIG. 2 exemplifies process of inserting a switch of the presentinvention between a host computer and an old disk apparatus;

FIG. 3 exemplifies a data flow during online data migration process ofthe present invention;

FIG. 4 exemplifies a data flow after online data migration process ofthe present invention;

FIG. 5 is a flowchart for explaining data migration process of thepresent invention;

FIG. 6 is a flowchart for explaining operational procedures ofconnecting the old disk apparatus of the present invention to theswitch;

FIG. 7 is a SCSI command kind table;

FIG. 8 is a flowchart for identifying whether a SCSI command from thehost computer of the present invention is “disk inherency” or not; and

FIG. 9 is a block diagram of disk apparatuses of the computer system ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following part, a computer system and an interface commandcontrol method of a disk apparatus of the present invention areexplained with reference to the embodiments of the present invention ofFIGS. 1 to 9.

(1) Computer System Diagram

FIG. 1A exemplifies a computer system diagram before data migration. Thehost computer 1 is connected to an old disk apparatus 2 via access paths3 and 4. The access paths 3 and 4 are fiber channels or parallel SCSIbased on the SCSI standard.

FIG. 1B exemplifies a computer system diagram after migration. A switch5 is connected to the host computer 1 via access paths 3 a and 4 a andthe old disk apparatus 2 via access paths 3 b and 4 b. The access paths3 a and 4 a and the access paths 3 b and 4 b are fiber channels and theparallel SCSI based on the SCSI standard. These access paths 3 a and 4 aand access paths 3 b and 4 b are access paths after switchingconnection.

Further, the switch 5 is connected to a new disk apparatus 6 via accesspaths 7 and 8. The access paths 7 and 8 are fiber channels or theparallel SCSI based on the SCSI standard. Here, the number of hostcomputers shall not be limited by the present invention, when thisinvention is made and used, since the present invention is datamigration from the old disk apparatus 2 to the new disk apparatus 6.

A host interface control part 9 is connected to the access paths 3 a and4 a via ports 15 and 16 and controls SCSI command transmitting to andreceiving from the host computer 1. Further, the host interface controlpart 9 is connected to disk interface control parts 13 and 14 and areserve emulation part 10 via internal paths 21, 29, and 22respectively.

A disk interface control part 13 is connected to the access paths 3 band 4 b via ports 17 and 18, and controls SCSI command transmitting toand receiving from the old disk apparatus 2. Further, a disk interfacecontrol part 13 is connected to the reserve emulation part 10, a SCSIcommand control part 11, and an online data migration part 12 viainternal paths 23, 25, and 27 respectively.

A disk interface control part 14 is connected to the access paths 7 and8 via ports 19 and 20 and controls SCSI command transmitting to andreceiving from the new disk apparatus 6. Further, the disk interfacecontrol part 14 is connected to the SCSI command control part 11 and theonline data migration part 12 via internal paths 26 and 28 respectively.

The reserve emulation part 10 receives SCSI commands, which are issuedby the host computer 1, from the host interface control part 9 via aninternal path 22 and executes exclusive access control to access fromthe host computer in accordance with the SCSI commands with regard toreserve and release (the number of host computers to be connected is notlimited, although one host computer 1 is indicated in FIG. 1B).

The SCSI command control part 11 receives the SCSI commands from thehost computer 1 via the host interface control part 9 and the reserveemulation part 10, and gives the SCSI commands to the old disk apparatus2 and the new disk apparatus 6 via the disk interface control part 13and the disk interface control part 14. Further, response informationfrom the old disk apparatus 2 and the new disk apparatus 6 is given tothe host computer 1 via the paths in reverse. The SCSI command controlpart 11 has the function of classifying the SCSI commands from the hostcomputer 1 and giving them to the old disk apparatus 2 or the new diskapparatus 6. The operation therefor is explained hereinafter withreference to FIGS. 3 and 4.

The online data migration part 12 of FIG. 1B has the function of copyingautomatically data stored in the disk apparatus, reads data via the diskinterface control part 13 from the old disk apparatus 2, and writes datavia the disk interface control part 14 in the new disk apparatus 6, whendata migration is executed.

(2) Interface Command Control Procedure for Data Migration

An example of data migration process of the computer system of FIGS. 1Aand 1B is explained using flowcharts of FIGS. 5 and 6. FIG. 5 indicatesprocess in which data migration is executed in an order of steps 51, 52,53, 54, and 55. They are expressed as (step 51), (step 52), . . . , and(step 55) in the latter part of the description. Further, FIG. 6indicates operator's operational procedures 61 to 64 which are executedat the step 52 of FIG. 5.

Before starting data migration, as shown in FIG. 1A, the host computer 1accesses the old disk apparatus 2 using the access paths 3 and 4. Firstof all, the operator operates the system so as to connect the new diskapparatus 6 to the switch 5 via the access paths 7 and 8 (step 51).

Next, in order to connect the old disk apparatus 2 to the switch 5, thesystem is operated in accordance with the procedures 61 to 64 of FIG. 6(step 52). The operator changes-over and connects the access path 4 tonew access paths 4 a and 4 b (the procedure 61 of step 52). At thistime, the host computer 1 detects changing-over of the access path 4,makes the access path 4 to an offline state, and continues to access theold disk apparatus 2 using the access path 3. Next, the host computer 1is operated so as to make the access path 4 a to an online state (theprocedure 62 of step 52).

FIG. 2 indicates the condition of this case. The host computer 1 and theold disk apparatus 2 are connected logically using SCSI data paths 30and 31. Next, the access path 3 is changed-over and connected to the newaccess paths 3 a and 3 b (the procedure 63 of step 52). Next, the hostcomputer 1 is operated so as to make the access path 3 a to the onlinestate (the procedure 64 of step 52). Accordingly, the host computer 1,the switch 5, and the old disk apparatus 2 are connected as indicated inFIG. 1B, and the host computer 1 accesses the old disk apparatus 2 viathe switch 5.

Next, in step 53 of FIG. 5, process in relation to SCSI reserve migratesto the reserve emulation part 10 from the old disk apparatus 2. To bemore precise, the reserve emulation part 10 accesses the old diskapparatus 2 via an internal path 23, checks the reserve condition of theold disk apparatus 2, sets its condition as an initial condition of thereserve emulation part 10, and then releases a reserve condition of theold disk apparatus 2. Then, process of SCSI commands with regard toreserve such as release and reserve received from the host computer andprocess of controlling access from another host computer (anotherinitiator) for a disk apparatus are executed in the reserve emulationpart 10. The reserve condition of the old disk apparatus in the processof this step 53 shall be released before online data migration (step 54)execution. Because, there may be caused trouble condition for a processof reading data from the old disk apparatus 2 when online data migrationis executed for the condition in which the old disk apparatus 2 isreserved by a reserve command from the host computer. Therefore, the olddisk apparatus can be accessed without reserving by including thereserve emulation part 10 in the upper portion of an online datamigration part 12.

Next, the online data migration part 12 copies data from the old diskapparatus 2 to the new disk apparatus 6 (step 54). Here, with referenceto FIG. 3, a typical data flow during data migration is explained. Thehost computer 1 and the old disk apparatus 2 are connected logically viaa SCSI data path 32, and the old disk apparatus 2 is continuouslyaccessed from the host computer 1. Further, on a parallel with thiscase, the online data migration part 12 copies data from the old diskapparatus 2 to the new disk apparatus 6 as indicated in a path 33 ofSCSI data.

Next, when online data migration process is completed, the SCSI commandcontrol part 11 changes-over SCSI commands, with regard to disk read andwrite, issued by the host computer 1 to the new disk apparatus 6 (step55). Here, with reference to FIG. 4, a typical data flow is explainedafter data migration. As indicated by a SCSI data path 35 of FIG. 4,SCSI commands, with regard to write and read, issued by the hostcomputer 1 is given to the new disk apparatus 6 via the disk interfacecontrol part 14 and an internal path 26 from the SCSI command controlpart 11. The SCSI commands, with regard to write and read, issued by thehost computer 1 are not issued to the old disk apparatus 2.

The SCSI command control part 11 identifies SCSI commands foridentifying the disk apparatuses issued by the host computer 1, i.e.,inquiry commands and SCSI commands in relation to mode sense series notspecified in the SCSI standard specification, and issues themcontinuously to the old disk apparatus 2 (SCSI data path 32 of FIG. 4).Accordingly, although process in relation to actual read and writeoperations is executed in the new disk apparatus 6, data migration canbe completed without stopping since the host computer 1 recognizes thatthe old disk apparatus 2 is continuously connected.

If suchlike data migration is executed by plural host computers, the oldmagnetic disk apparatus is shared by plural host computers. Further, ifplural old magnetic disk apparatuses are connected, plural magnetic diskapparatuses are shared.

FIG. 7 is a SCSI command kind table. In this table, SCSI commands forSCSI command names “Inquiry” (operation codes 12H) and “Mode Sense”(operation codes 1AH) series not specified in the SCSI standardspecification are command kinds which shall be set in the disk apparatusinherently. For accessing data after data migration between magneticdisk apparatuses, there has been anxiety in which data access might benegatived after migration by recognizing suchlike command kindinconsistency as disk exchange information disagreement by the hostcomputer. However, in accordance with is the computer system and aninterface command control method of the present invention, it wasrecognized that suchlike inconsistent recognition was avoided and dataaccess after migration was executed steadily.

FIG. 8 indicates specific procedures for a method of controllinginterface commands of the present invention for avoiding thisinconsistent recognition.

FIG. 9 illustrates a condition in which inherent information is storedas memory, which corresponds to each command kind set in the inherentmagnetic disk apparatus of the computer system of the present invention.

In the above mentioned example, the case of utilizing the switch 5 fordata migration is explained. The present invention is not limited tothis case, but it is also applicable to execute data migration byutilizing the disk apparatus or disk controller (generic term ofportions including a disk control part 205, CPU 204, a port control part203, a memory 202, and inherent information 201 of the old diskapparatus of FIG. 9).

As explained in the above cited, the present invention does not need anaccess stop to the disk apparatus from the host computer for datamigration procedures from the old disk apparatus to the new diskapparatus, and thus non-stop data migration can be made.

Further, fault condition caused by disk apparatus inherent informationchange when data migration is executed from the old disk apparatus tothe new disk apparatus can be avoided and thus the disk apparatus havinginconsistent specification can be exchanged without any stopping.

Further, redundancy of access paths among the host computer, the switch,and the disk apparatus can be maintained and thus highly availablecomputer system data migration and operational availability after datamigration can be made.

Further, complete non-stop system migration can be made by including thefunction of simulating reserve process even for the disk apparatusshared by plural host computers.

In this way, the invention is explained concretely based on theembodiments invented by the inventor. The present invention is notlimited to the above cited embodiments, but it goes without saying thatvarious modifications can be applied within the scope of the gist of thepresent invention.

Although data migration has been exemplified as the function of theswitch, for example, this data migration can also be applied widely to adisk array (RAID) subsystem including the equivalent function. Further,the old disk apparatus after data migration can be diverted to datastorage used by the switch and the disk array sub system.

According to the data migration method of the disk apparatus of thepresent invention, since the host computer recognizes that the old diskhas been connected even during or after executing data migration to thenew disk apparatus from the old disk apparatus, effects, in which thehost computer does not need to change the definition of the diskapparatus and then tasks on the host computer can be performedcontinuously, are obtained.

Further, in a system in which the host computer is connected to the olddisk apparatus via plural redundant access paths, since the hostcomputer can access the disk apparatus without stopping when datamigrate and the host computer can also utilize plural access pathscontinuously after data migration, effects, in which data migration canbe executed without stopping access and protection against the fault ofaccess paths can be maintained, are obtained.

Furthermore, in a cluster system in which plural hosts share the olddisk, the effect in which non-stop data can migrate smoothly while thecluster is maintained since exclusion information of the disk apparatusalso migrates is obtained.

Furthermore, after data migration, the effect in which the old disk canbe diverted to the device for storing other data is obtained.

1. A method of controlling data transfer in a switch system coupled to ahost system and a plurality of storage devices, the host system usinglogical unit identification information of a logical unit to access theswitch system, and each of the storage devices having a plurality ofdisk drives, the method comprising: a first status comprising: receivingdata of a first data input request, the first data input request beingsent from the host system and being targeted to the logical unitidentification information, and transferring the data of the first datainput request to a first storage device of the storage devices so thatthe first storage device inputs the data of the first data input requestinto a storage area of the disk drives of the first storage device; asecond status comprising: transferring data stored in the first storagedevice to a second storage device of the storage devices, a third statuscomprising: receiving data of a second data input request, the seconddata input request being sent from the host system and being targeted tothe logical unit identification information, and transferring the dataof the second data input request to the second storage device so thatthe second storage device inputs the data of the second data inputrequest into a storage area of the disk drives of the second storagedevice, wherein the second status is conducted between the first statusand the third status, and wherein the switch system controls to allow ordeny to authorize a reserve to the host system based on whether anotherhost system has the reserve authority of the logical unit of the logicalunit identification information.
 2. The method of controlling datatransfer according to claim 1, wherein the switch system controls toallow or deny to authorize the reserve to the host system during atleast the second status.
 3. The method of controlling data transferaccording to claim 1, wherein the switch system controls to allow ordeny to authorize the reserve to the host system based on a command, thecommand being used to request a reserve and being sent from the hostsystem.
 4. The method of controlling data transfer according to claim 1,wherein the switch system controls to allow to authorize the reserve tothe host system and controls to release the logical unit of the logicalunit identification information from the host system.
 5. The method ofcontrolling data transfer according to claim 1, wherein the switchsystem controls to allow to authorize the reserve to the host systembased on a first command sent from the host system and controls torelease the logical unit of the logical unit identification informationfrom the host system based on a second command sent from the hostsystem.
 6. The method of controlling data transfer according to claim 1,wherein the switch system controls to allow or deny to authorize thereserve to the host system based on a Reserve command of SCSI standards.7. The method of controlling data transfer according to claim 1, whereinthe switch system receives and controls a Test Unit Ready command ofSCSI standards.
 8. The method of controlling data transfer according toclaim 1, wherein the switch system receives and controls a Rezero Unitcommand of SCSI standards.
 9. The method of controlling data transferaccording to claim 1, wherein the switch system receives and controls aRequest Sense command of SCSI standards.
 10. The method of controllingdata transfer according to claim 1, wherein the switch system receivesand controls a Format command of SCSI standards.
 11. The method ofcontrolling data transfer according to claim 1, wherein the switchsystem receives and controls a Inquiry command of SCSI standards. 12.The method of controlling data transfer according to claim 1, whereinthe switch system receives and controls a Verify command of SCSIstandards.
 13. A method of controlling data transfer in a switch systemcoupled to a host system and a plurality of storage devices, the hostsystem using an identification information to access the switch system,and each of the storage devices having a plurality of disk drives, themethod comprising: receiving data of a first data input request, thefirst data input request being sent from the host system and beingtargeted to the identification information; transferring the data of thefirst data input request to a first storage device of the storagedevices so that the first storage device inputs the data of the firstdata input request into a storage area of the disk drives of the firststorage device; transferring data stored in the first storage device toa second storage device of the storage devices; receiving data of asecond data input request, the second data input request being sent fromthe host system and being targeted to the identification information;and transferring the data of the second data input request to the secondstorage device so that the second storage device inputs the data of thesecond data input request into a storage area of the disk drives of thesecond storage device; and controlling to allow or deny to reserve alogical unit for the host system based on whether the logical unit isreserved for another host system.
 14. The method of controlling datatransfer according to claim 13, wherein the switch system controls toallow or deny to reserve the logical unit for the host system during atleast the transferring data stored in the first storage device to thesecond storage device.
 15. The method of controlling data transferaccording to claim 13, wherein the switch system controls to allow ordeny to reserve the logical unit for the host system based on a command,the command being used to request a reserve and being sent from the hostsystem.
 16. The method of controlling data transfer according to claim13, wherein the switch system controls to allow to reserve the logicalunit for the host system and controls to release the logical unit fromthe host system.
 17. The method of controlling data transfer accordingto claim 13, wherein the switch system controls to allow to reserve thelogical unit for the host system based on a first command sent from thehost system and controls to release the logical unit from the hostsystem based on a second command sent from the host system.
 18. Themethod of controlling data transfer according to claim 13, wherein theswitch system controls to allow or deny to reserve the logical unit forthe host system based on a Reserve command of SCSI standards.
 19. Themethod of controlling data transfer according to claim 13, wherein theswitch system receives and controls a Test Unit Ready command of SCSIstandards.
 20. The method of controlling data transfer according toclaim 13, wherein the switch system receives and controls a Rezero Unitcommand of SCSI standards.
 21. The method of controlling data transferaccording to claim 13, wherein the switch system receives and controls aRequest Sense command of SCSI standards.
 22. The method of controllingdata transfer according to claim 13, wherein the switch system receivesand controls a Format command of SCSI standards.
 23. The method ofcontrolling data transfer according to claim 13, wherein the switchsystem receives and controls a Inquiry command of SCSI standards. 24.The method of controlling data transfer according to claim 13, whereinthe switch system receives and controls a Verify command of SCSIstandards.
 25. A method of controlling data transfer in a switch systemcoupled to a host system and a plurality of storage devices, the hostsystem using an identification information to access the switch system,and each of the storage devices having a plurality of disk drives, themethod comprising: receiving data of a first data input request, thefirst data input request being sent from the host system and beingtargeted to the identification information; transferring the data of thefirst data input request to a first storage device of the storagedevices so that the first storage device inputs the data of the firstdata input request into a storage area of the disk drives of the firststorage device; transferring data stored in the first storage device toa second storage device of the storage devices; receiving data of asecond data input request, the second data input request being sent fromthe host system and being targeted to the identification information;transferring the data of the second data input request to the secondstorage device so that the second storage device inputs the data of thesecond data input request into a storage area of the disk drives of thesecond storage device; and controlling to allow or deny to reserve astorage target for the host system.
 26. The method of controlling datatransfer according to claim 25, wherein the switch system controls toallow or deny to reserve the storage target for the host system duringat least the transferring data stored in the first storage device to thesecond storage device.
 27. The method of controlling data transferaccording to claim 25, wherein the switch system controls to allow ordeny to reserve the storage target for the host system based on acommand, the command being used to request a reserve and being sent fromthe host system.
 28. The method of controlling data transfer accordingto claim 25, wherein the switch system controls to allow or deny toreserve the storage target for the host system and controls to releasethe logical unit from the host system.
 29. The method of controllingdata transfer according to claim 25, wherein the switch system controlsto allow or deny to reserve the storage target for the host system basedon a first command sent from the host system and controls to release thelogical unit from the host system based on a second command sent fromthe host system.
 30. The method of controlling data transfer accordingto claim 25, wherein the switch system controls to allow or deny toreserve the storage target for the host system based on a Reservecommand of SCSI standards.
 31. The method of controlling data transferaccording to claim 25, wherein the switch system receives and controls aTest Unit Ready command of SCSI standards.
 32. The method of controllingdata transfer according to claim 25, wherein the switch system receivesand controls a Rezero Unit command of SCSI standards.
 33. The method ofcontrolling data transfer according to claim 25, wherein the switchsystem receives and controls a Request Sense command of SCSI standards.34. The method of controlling data transfer according to claim 25,wherein the switch system receives and controls a Format command of SCSIstandards.
 35. The method of controlling data transfer according toclaim 25, wherein the switch system receives and controls a Inquirycommand of SCSI standards.
 36. The method of controlling data transferaccording to claim 25, wherein the switch system receives and controls aVerify command of SCSI standards.